In the manufacture of composite laminate panels, thermosetting resins such as phenolics, polyesters and other reactive thermosets have been used as matrix materials to make plies of composite fiber-resin material. Generally, prepregs (a common designation for preimpregnated items of manufacture for use in subsequent manufacturing steps) incorporating plies of reinforcing fibers or fabrics wet out with reactive thermosetting resins in liquid form and stacked on top of one another are subjected to pressure and heat. The stacked material is usually subjected to a curing cycle where the heat-curable thermosetting resins are cured or set up to make the final structure. Any material that must be trimmed is scrapped since the thermosetting resins cannot be recycled back into the production process. Handling reactive (i.e., curable) liquids is problematic due to the possibility of spills, contamination and operator contact. The thermosetting resins and dust therefrom sometimes present exposure hazards to workers, and disposal of the reactive thermosetting material is problematic. When a phenol matrix material is used in a composite laminate panel such as a ballistic panel, the panel typically needs to be cured at 2100 psi for about an hour. In addition, the panel must be de-gassed to avoid the formation of voids (bubbles) in the panel. De-gassing leads to the release of volatile organic compounds (VOCs), which presents environmental concerns.
In contrast to the foregoing problems associated with thermoset matrix composites, composite laminates employing a thermoplastic matrix are easier, cleaner and simpler to handle and produce. Any waste material can be easily reworked into the process since the thermoplastic resins used do not cure or crosslink during processing, molding or heating. No special storage is required and shelf life of a thermoplastic based material is virtually indefinite, making in-process inventory of impregnated fiber sheets usable without regard to when they were manufactured. Moreover, the mechanical properties of a thermoplastic vary greatly as compared with a thermoset material. For example, thermoset materials are often hard and brittle while thermoplastics can be more pliable and subject to easier post-processing. This variation in properties coupled with the added strength resulting from the reinforcing fibers embedded in the thermoplastic matrix provide a more versatile composite material.
Composite laminates employing a thermoplastic matrix have numerous uses, including the manufacture of panels for use in armor.
Armor, or ballistic material, for vehicles and personnel is finding ever increasing application in modern times. With respect to vehicles, armor has historically taken the form of metal plates, the thickness of which varies depending on the type of projectile the armor is designed to stop. As this metal armor gets thicker, the weight of the armor increases dramatically. Making the metal armor thinner while reducing weight will likewise reduce the ability of the armor to stop the intended projectile.
Other engineered materials, such as ceramics, have been employed as armor. However, these materials are also heavy and can be prohibitively expensive. Moreover, these materials are often difficult to form and can require costly molds and dies. In aircraft where minimizing weight is critical, it is sometimes impossible to use any armoring material. This leaves personnel and equipment subject to severe injury and damage.
Many different types of armor are now available that range in resistance from those designed to protect against small caliber handguns to those designed to protect against high-powered rifles. Ballistic materials are used to fabricate portable ballistic shields, such as a ballistic clipboard for use by a police officer; to provide ballistic protection for fixed structures such as control rooms or guard stations; and to provide ballistic protection for the occupants of vehicles. Different types of ballistic materials can be used alone or in combination with one another depending on the intended threat protection.
Ballistic materials (sometimes referred to herein as “ballistic panels”), are usually tested in accordance with standards that allow for consistent and meaningful evaluation of their performance, i.e., their ability to withstand ballistic impact. Such a standard has been established by the United States Department of Justice's National Institute of Justice and is entitled “NIJ Standard for Ballistic Resistant Protective Materials” (hereinafter referred to as the “NIJ Standard”). The NIJ Standard is incorporated herein by reference. The ballistic threat posed by a bullet or other projectile depends, inter alia, on its composition, shape, caliber, mass, and impact velocity. Accordingly, the NIJ standard has classified the protection afforded by different armor grades as follows.
Type II-A (Lower Velocity 357 Magnum and 9 mm): Armor classified as Type II-A protects against a standard test round in the form of a 357 Magnum jacketed soft point, with nominal masses of 10.2 g and measured velocities of 381+/−15 meters per second. Type II-A ballistic materials also protect against 9 mm full metal jacketed rounds with nominal masses of 8 g and measured velocities of 332+/−12 meters per second.
Type II (Higher Velocity 357 Magnum; 9 mm): This armor protects against projectiles akin to 357 Magnum jacketed soft point, with nominal masses of 10.2 g and measured velocities of 425+/−15 meters per second. Type II ballistic materials also protect against 9 mm full metal jacketed rounds with nominal masses of 8 g and measured velocities of 358+/−12 meters per second.
Type III-A (44 Magnum, Submachine Gun 9 mm): This armor provides protection against most handgun threats, as well as projectiles having characteristics similar 44 Magnum, lead semiwadcutter with gas checks, having nominal masses of 15.55 g and measured velocities of 426+/−15 meters per second. Type III-A ballistic material also protects against 9 mm submachine gun rounds. These bullets are 9 mm full metal jacketed with nominal masses of 8 g and measured velocities of 426+/−15 meters per second.
Type III (High Powered Rifle): This armor protects against 7.62 mm (308 Winchester®) ammunition and most handgun threats.
Type IV (Armor-Piercing Rifle): This armor protects against 30 caliber armor piercing rounds with nominal masses of 10.8 g and measured velocities of 868+/−15 meters per second.
Other threats recognized in the art include Improvised Explosive Devices (IEDs), which may generate shrapnel that may be only a few grains in weight and may have velocities up to 5000 ft/sec.
In addition to the foregoing standards, criteria such as the percentage of projectiles allowed to penetrate a particular ballistic material are also employed in evaluating ballistic materials. One such test is the V50 test as defined by MIL-STD-622, V50 Ballistic Test for Armor. According to this test, the final state of a witness plate placed behind the armor panel determines the experimental outcome of the ballistic test as shown in FIG. 1A and FIG. 1B. FIGS. 1A and 1B illustrate two situations occur as a result of the ballistic test: FIG. 1A illustrates partial penetration of the test panel 10, evidenced by lack of perforation of the “witness plate” 12; and FIG. 1B illustrates complete penetration of test panel 10, evidenced by visibility of light through the witness plate 12 by a projectile or spall from the test panel 10. The area corresponding to a velocity range causing a mixture of partial and complete penetration is the Zone of Mixed Results (ZMR).
The V50 may be defined as the average of an equal number of highest partial penetration velocities and the lowest complete penetration velocities which occur within a specified velocity spread. A 0.020 inch (0.51 mm) thick 2024-T3 sheet of aluminum is placed 6±½ inches (152±12.7 mm) behind and parallel to the target to witness complete penetrations. Normally at least two partial and two complete penetration velocities are used to compute the V50 value. Four, six, and ten-round ballistic limits are frequently used. The maximum allowable velocity span is dependent on the armor material and test conditions. Maximum velocity spans of 60, 90, 100, and 125 feet per second (ft/s) (18, 27, 30, and 38 m/s) are frequently used.
A ballistic material commonly used as a comparative reference for V50 tests is known in the art as HJ1, and is known to comprise woven S-glass fibers in a phenol-type thermosetting matrix material.
Another known ballistic material used as personal body armor comprises Kevlar aramid fabric that has been bonded with polyethylene in a process that is insufficient for the Kevlar fabric fibers to be encapsulated by the polyethylene.